The conventional practice of purging water during the process of sampling water found in a groundwater monitoring well is based on the need to remove stagnant water from the portion of the well that is not open to the water bearing formation prior to sampling. It is thought that this stagnant water is not representative of the in-situ water quality and, therefore, must be removed before any type of accurate sampling process can be allowed to proceed. Traditional practices of bailing or high-rate pumping of water from a well during the purging process necessitate the removal of multiple “well volumes” to ensure adequate removal of all stagnant water from the sampling zone. For instance, in a known procedure for sampling ground water monitoring wells, three to five times the volume of the standing water in a well screen, casing and surrounding filter pack is removed prior to collecting a sample to assure that only unaltered formation water is tested.
The use of traditional sampling devices and pumping systems, to conduct groundwater monitoring results in a time consuming and costly sampling process. The use of samplers, such as bailers, portable pumps, and high speed pumps mixes the stagnant water in the well casing with the “fresh” water in the screened interval. Moreover, high-rate pumping of water from the well can induce drawdown within the well and surrounding formation, causing stagnant water to be mixed with incoming water and contaminating the samples. High-volume purging often requires hours to complete, and results in the creation of tens or hundreds of gallons of purge water which often must be contained for disposal as a liquid hazardous waste.
The cost of properly disposing of groundwater that qualifies as a hazardous material further adds to the cost of the sampling process. It would therefore be desirable to minimize the amount of water that is required to be removed prior to collecting the water sample, yet still provide a representative sample.
Several methods have been proposed to deal with this specific problem. One of the methods that has been gaining acceptance in the groundwater monitoring community is the “low flow” sampling process. Low flow sampling is the process of pumping a monitoring well at a very low flow rate before sampling to eliminate the mixing of stagnant water above the screened interval with the fresh water in the screened interval. During low flow sampling, a very limited volume of water is removed from the well, resulting in minimal drawdown of the water column within the well. Low flow sampling is based on the rationale that a screened section of a well casing has fresh formation water flowing through it at a rate equal to the rate it moves through the aquifer. Water in a non-screened section of the well casing, above the screened section, is stagnant and thus not representative of in-situ well water conditions. Low flow sampling focuses on removing water only from the screened interval.
The low flow sampling process requires that a dedicated pump be left in the well for sampling purposes. The pump is lowered into the well and is positioned in the screened section of the well. As the pump is lowered downwardly through the well, it first passes through the stagnant water existing in the non-screened section of the well casing. As it is lowered, it mixes the stagnant water with the fresh formation water in the screened zone of the well below. Because of this initial mixing, the pump must sit undisturbed within the screened section of the well until the well once again reaches equilibrium. After equilibrium is reached, usually in a matter of days, sampling can begin. The pump is started and water is withdrawn from the well at a rate which does not result in draw down of the water level and mixing of the stagnant water into the well screened section. As the water is slowly pumped from the well, it is monitored for stabilization of indicator parameters. As soon as the parameters are stabilized, indicating that the pumping system has been purged of extraneous water and that the withdrawn water is coming from the formation, a sample is collected.
The disadvantages of the low flow sampling process are that it is often slow and complex and requires specialized training of the sampling personnel. Moreover, the capital costs associated with the equipment used to conduct low flow sampling is high. The dedicated pump and tubing are costly and additional surface instrumentation further adds to the cost of the procedure. Accordingly, there is a need for a sampling device that is quick and easy to use that does not require extensive training of the field personnel. Moreover, it is desirable to have a sampling device that does not require costly equipment to operate.
Another known sampling method that has been proposed is the “no purge” sampling method that is carried out by using a conventional sampler. Like the low flow sampling method, the no purge concept is based on the premise that a well's intake screen and filter pack are more permeable than the formation being sampled, and that because of this, water is constantly flowing through the well's screened section. If there is no stagnant water in the well casing, as when the top of the screened section is positioned above the top of the water table, it is proposed that purging is not needed. A conventional bailer is dropped into the water column and a sample of groundwater is immediately collected. To successfully use this procedure, however, the screened section of the well casing must extend above the water table, thereby eliminating the presence of stagnant water that could mix with the formation water as the bailer is lowered. The absence of such stagnant water eliminates the prospect of the recovered bailed water sample becoming contaminated.
A newer sampling method is “diffusion” sampling. Diffusion sampling is a passive sampling process that is conducted by using a sealed polyethylene bag filled with water that is lowered into the screened section of a monitoring well. Molecular diffusion of volatile organic contaminants (VOC's) causes chemical equilibrium to occur between the water in the sampler and the water in the well. After allowing approximately 14 days for the concentrations to equilibrate, the sampler is withdrawn and the water in the bag is removed and analyzed. This method has limited applicability, however, as only specific types of contaminants diffuse through the polyethylene bag and different contaminants diffuse at different rates. Accordingly, a need exists for a sampling device that has broad applicability